The popularity of LEDs in various applications where illumination is needed is on the rise. Although traditionally utilized for indicator lights, background display illumination and other applications where low-level illumination is sufficient, the realization of high brightness blue/green and violet LEDs made from the III-nitride semiconductor family (such as InN, GaN, AlN and their alloys) has dramatically increased the utility of LEDs in providing general illumination for a variety of settings, including residential houses and commercial buildings. Because solid-state lighting LEDs (SSL-LEDs or LED lamps) are quite efficient in light output per watt of power consumed, and are sufficiently versatile to be capable of use in a variety of locations where illumination is needed, LED lamps have the potential to replace traditional incandescent or fluorescent lamps in many lighting applications. The use of LED lamps, therefore, will enable overall electrical energy consumption to be reduced, while providing a very durable lighting source that is virtually maintenance free. Furthermore, materials used to form LED chips and associated coatings allow LED lamps to be independently tunable in terms of light intensity and visible spectrum color output.
Because traditional LED illuminators operate under a low DC voltage supply, these LEDs have required voltage down-conversion transformers and other circuitry to operate in general lighting applications where a standard AC power source of 110/120 or 220/240 volts is supplied (e.g., in a residence or other building). To overcome the need for such circuitry, while achieving a high luminance for general lighting applications, an LED lamp has been developed that runs on high input power generated by high DC or AC voltage. One implementation of a high AC voltage LED lamp is disclosed in U.S. Pat. No. 6,957,899, issued on Oct. 25, 2005, and entitled “Light Emitting Diodes For High AC Voltage Operation and General Lighting”. The '899 patent teaches integrating many individual E-Nitride LEDs onto a single chip or wafer grown, for instance, on an electrically insulating sapphire substrate, with serial interconnection between the LEDs to achieve a single light emitting device, or lamp. The LED lamp can be configured to have a running voltage of 12V, 24V, 110/120V, 220/240V, or other values. This is achieved in the invention of the '899 patent by having two columns of individual LEDs each in serial interconnection and wired in opposing polarities such that one column of LEDs is forward biased and the other column is reverse biased.
FIGS. 1a-1c illustrate an exemplary high voltage AC or DC powered SSL-LED lamp, such as the LED lamp of the '899 patent. With particular attention to FIG. 1a, the first phase of an LED buildup 10 or layered structure is shown. The individual layers forming the LED buildup 10 are deposited onto a generally electrically insulating substrate 12. More specifically, a buffer layer 14 with a typical thickness of about 0.05 micron of semiconductor material is first deposited onto the substrate 12, followed by a layer 16 of n-type semiconductor material, in one example, a 2-4 μm layer of n-type gallium nitride (n-GaN). An optically active region 18 is then formed by depositing a layer of light-emission material onto the n-type semiconductor layer 16, such as indium gallium nitride/gallium nitride multiple quantum wells (InGaN/GaN MQW), followed by a layer 20 of p-type semiconductor material, such as p-GaN, deposited onto the optically active region 18. FIG. 1b shows the LED layered structure that has been etched down (e.g., through dry or wet chemical etching) to the insulating substrate 12, thereby forming an isolated mesa structure of multiple discrete LEDs 22 spaced across the substrate 12. Each LED 22 also has both the n-type 16 and p-type layers 20 exposed so that a conductive trace 24 (e.g., deposited Ni/Au metal stacks) may be extended between a p-type ohmic contact 26 of one LED 22 and an n-type ohmic contact 28 of an adjacent LED 22, as depicted in FIG. 1c. An insulating material 30, such as silicon oxide, is disposed along a wall of the layered buildup beneath the conductive trace 24 to isolate the n-type layer 16 from the p-type layer 20 of the same LED 22, which would short circuit the LED 22 and inhibit electrical interconnection of the LEDs across an SSL-LED lamp 32.
As can be understood, the SSL-LED lamp 32 of FIGS. 1a-1c is fabricated onto the electrically insulating substrate 12 in order to prevent an improper conductive path being formed between the n-type layers 16 of adjacent LEDs 22 through the substrate 12 and the respective buffer layers 14, which would result if the substrate was formed of a material possessing a sufficiently high level of electrical conductivity. As one example, for LEDs formed by GaN-based semiconductor growth, sapphire or silicon carbide (SiC) substrates have been widely used. Both of these substrate types have disadvantages. Sapphire substrates have low thermal conductivity, which inhibits the dissipation of heat generated when an LED is in operation. This excessive heat reduces the light output of individual LEDs and shortens the lifespan of the lamp. In the case of silicon carbide substrates, a high thermal conductivity is realized. SiC substrates also provide a lower lattice mismatch with GaN-based semiconductors as compared to a sapphire substrate, resulting in GaN-based semiconductors possessing high crystal quality and strong illumination performance for the underlying LED. Still, SiC is a somewhat electrically conductive, and thus is a less than ideal substrate for building an LED lamp. It has also been suggested to use Silicon alone as a substrate, because it is relatively inexpensive and provides a foundation onto which an integrated protection circuit for LEDs can be built. Unfortunately, silicon is also somewhat conductive, and thus is disadvantageous for the reasons stated above. The same issue with semi conductive substrates arises with AlInGaP-based LEDs, where typical substrates formed of GaAs and InP are also semiconductors.
Turning to AlInGaN-based LEDs, additional problems arise because of the frequent need to provide an etching depth of 4 μm or more when forming the isolated mesa structure of multiple individual LEDs 22, as shown in FIG. 1b. This etching depth creates deep trenches between adjacent LEDs 22. These deep trenches make it difficult to properly form the conductive traces 24 that extend between the p-n junctions of the LEDs 22 (as shown in FIG. 1c), leading to poor quality electrical isolation of n-type layers 16 of adjacent LEDs 22 and an increase in electrical resistance within the SSL-LED lamp 32. Therefore, it is desirous to improve LED construction to provide reliable and consistent performance in illumination applications where relatively high voltage AC or DC power is supplied.